Unlock Piecemeal
Beat the game on hard with either Trapper or every other character.

Unlock Axel
In One Player mode, select Tournament mode. In the Playground Peril level, go to where you see a swing set next to a boy on a merry-go-round in the background. Shoot a missile at the middle swing and a figure of Axel will be thrown into the playground. Pick it up to unlock him.

Darkside:
In the Easy Death Oven level in tournament mode, go to the sink and drive down the plumber's butt. Go onto his leg and napalm his butt. He will blow a hole in the wall. Enter the hole to find the icon that unlocks Darkside.

Mime:
In the Holiday Havoc level in the medium difficulty setting in endurance mode, shoot missiles at the Christmas tree until it is destroyed. Get on top of the fireplace then jump off backwards and shoot a missile at the stockings. Destroy all four to reveal the icon that unlocks Mime.

Piecemeal:
Successfully complete the game as Trapper to unlock Piecemeal as a playable character.

Trapper:
Successfully complete the game as Mime to unlock Trapper as a playable character.

Shock Therapy level:
Get ten kills in an endurance match.

Buster's Lanes level:
Get twenty kills in an endurance match.

Holiday Havoc level:
Get thirty kills in an endurance match.

Playground Peril: Hidden health:
Shoot the big duck to get a health power-up.

Playground Peril: Shoot boy on merry-go-round:
You can shoot missiles at the boy on the merry-go-round from the playhouse. He will spin so fast that he will fly off.